This project concerns genetic control of cell interactions during development of the mammalian central nervous system. The mouse, both normal and mutant, serves as the experimental material. The methods are interdisciplinary, involving light and electron microscopy, tissue culture, chemistry, cellular immunology, electrophysiology, and genetics. The primary focus of attention is on mutations that affect development of the cerebellum, where we have access to diseases causing selective destruction or failure of formation of each of several of the major neuronal types. The principles of gene action on brain development that are emerging from these studies will influence medical concepts of human mental retardation.